U.S. patent number 6,146,420 [Application Number 08/988,142] was granted by the patent office on 2000-11-14 for osteogenic fusion device.
This patent grant is currently assigned to SDGI Holdings, Inc.. Invention is credited to William F. McKay.
United States Patent |
6,146,420 |
McKay |
November 14, 2000 |
**Please see images for:
( Certificate of Correction ) ** |
Osteogenic fusion device
Abstract
An interbody osteogenic fusion device is provided that includes
opposite end pieces with an integral central element. The end
pieces are sized to maintain the height of an intervertebral disc
space. The central element has a much smaller diameter so that the
osteogenic fusion device forms an annular pocket around the central
element. An osteogenic material is disposed within the annular
pocket between the opposite end pieces. In one embodiment, the
osteogenic material constitutes a collagen sheet soaked in a
solution containing a bone morphogenetic protein. The osteogenic
fusion device is configured so that the osteogenic material is in
direct contact with the adjacent vertebral bone. In addition to the
enhanced area of contact between the vertebral bone and the fusion
material, the inventive osteogenic fusion device reduces
stress-shielding and minimizes the radio-opacity of the implant so
that growth of the fusion mass can be continuously assessed.
Inventors: |
McKay; William F. (Memphis,
TN) |
Assignee: |
SDGI Holdings, Inc. (Memphis,
TN)
|
Family
ID: |
25533886 |
Appl.
No.: |
08/988,142 |
Filed: |
December 10, 1997 |
Current U.S.
Class: |
623/17.16;
606/86A; 606/908; 606/909; 606/910; 606/912; 606/247 |
Current CPC
Class: |
A61F
2/4611 (20130101); A61F 2/446 (20130101); A61F
2/30767 (20130101); A61F 2002/30604 (20130101); A61F
2210/0004 (20130101); A61F 2230/0028 (20130101); A61F
2310/00017 (20130101); A61F 2002/30125 (20130101); A61F
2002/30293 (20130101); A61F 2002/448 (20130101); A61F
2230/0006 (20130101); A61F 2002/3082 (20130101); A61F
2230/0091 (20130101); A61F 2002/304 (20130101); A61F
2002/30677 (20130101); A61F 2230/0067 (20130101); Y10S
606/909 (20130101); A61F 2/28 (20130101); A61F
2002/30383 (20130101); Y10S 606/91 (20130101); A61F
2002/30169 (20130101); A61F 2/4603 (20130101); A61F
2002/30116 (20130101); A61F 2002/30235 (20130101); Y10S
606/908 (20130101); Y10S 606/912 (20130101); A61F
2002/3021 (20130101); A61F 2220/0041 (20130101); A61F
2002/30166 (20130101); A61F 2002/30131 (20130101); A61F
2230/0023 (20130101); A61F 2002/30787 (20130101); A61F
2220/0025 (20130101); A61F 2230/0008 (20130101); A61F
2310/00023 (20130101); A61F 2002/30774 (20130101); A61F
2002/30329 (20130101); A61F 2002/30777 (20130101); A61F
2002/3085 (20130101); A61F 2230/0013 (20130101); A61F
2230/0047 (20130101); A61F 2310/00161 (20130101); A61F
2310/00179 (20130101); A61F 2002/30433 (20130101); A61F
2002/30156 (20130101); A61F 2002/30224 (20130101); A61F
2002/30873 (20130101); A61F 2002/2817 (20130101); A61F
2230/0069 (20130101); A61F 2002/30593 (20130101); A61F
2250/0063 (20130101); A61F 2002/30225 (20130101); A61F
2002/30785 (20130101); A61F 2002/30616 (20130101); A61F
2310/00365 (20130101); A61F 2/30744 (20130101); A61F
2002/2835 (20130101); A61F 2002/30599 (20130101); A61L
2430/38 (20130101); A61F 2002/30062 (20130101); A61F
2/442 (20130101) |
Current International
Class: |
A61F
2/44 (20060101); A61F 2/46 (20060101); A61F
2/02 (20060101); A61F 2/28 (20060101); A61F
2/00 (20060101); A61F 2/30 (20060101); A61F
002/44 () |
Field of
Search: |
;623/16,17
;606/61,71-73 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
577179 |
|
May 1994 |
|
EP |
|
94/07441 |
|
Apr 1994 |
|
WO |
|
Primary Examiner: Snow; Bruce
Attorney, Agent or Firm: Woodard, Emhardt, Naughton Moriarty
& McNett
Claims
What is claimed is:
1. An implant for promoting fusion bone growth in the space between
upper and lower adjacent vertebrae, comprising:
a load bearing member including opposite end pieces and a rigid
elongated central element extending between said end pieces,
said opposite end pieces each having two opposite surfaces
configured to contact and support the upper and lower adjacent
vertebrae, said opposite end pieces sized to maintain the space
between the adjacent vertebrae,
said central element being sized relative to said opposite end
pieces to define a pocket between said central element and the
adjacent vertebrae when the adjacent vertebrae are supported by
said opposite end pieces, said pocket configured to contain an
osteogenic material disposed about said central element and in
intimate contact with the adjacent vertebrae when the vertebrae are
supported by said opposite end pieces; and
a biocompatible sheet carrier for an osteogenic material, said
biocompatible sheet carrier wound around said central element
within said pocket.
2. The implant for promoting fusion bone growth according to claim
1, wherein said opposite end pieces include:
a first end piece having a first dimension between said two
opposite surfaces, said first dimension being sized to maintain the
space between the adjacent vertebrae; and
a second end piece having a dimension between said two opposite
surfaces substantially equal to said first dimension and a second
dimension transverse to said dimension that is less then said first
dimension.
3. The implant for promoting fusion bone growth according to claim
1, wherein each of said two opposite surfaces of each of said
opposite end pieces is arcuate.
4. The implant for promoting fusion bone growth according to claim
3, wherein said opposite end pieces include:
a substantially circular first end piece; and
a second end piece with said two opposite surfaces being circular
and including truncated non-circular sides between said two
opposite surfaces.
5. The implant for promoting fusion bone growth according to claim
1, wherein:
each of said opposite end pieces defines a first dimension between
said two opposite surfaces; and
said elongated central element defines a central dimension
transverse to said longitudinal axis that is less than said first
dimension.
6. The implant for promoting fusion bone growth according to claim
5, wherein said central dimension is no more than about 25%
(twenty-five percent) of said first dimension.
7. The implant for promoting fusion bone growth according to claim
1, further comprising an osteogenic material contained within said
pocket and arranged to contact the adjacent vertebrae when the
vertebrae are supported by said opposite end pieces.
8. The implant for promoting fusion bone growth according to claim
7, wherein said osteogenic material includes an osteogenic
substance disposed within said biocompatible sheet carrier.
9. The implant for promoting fusion bone growth according to claim
1, further comprising an osteogenic material contained within said
pocket and arranged to contact the adjacent vertebrae when the
vertebrae are supported by said opposite end pieces.
10. The implant for promoting fusion bone growth according to claim
8, wherein said osteogenic substance is a bone morphogenetic
protein.
11. The implant for promoting fusion bone growth according to claim
1, wherein said two opposite surfaces of at least one of said end
pieces includes threads.
12. The implant for promoting fusion bone growth according to claim
1, wherein each of said opposite end pieces defines a retaining
surface connected to said central element, at least one retaining
surface being concave.
13. The implant for promoting fusion bone growth according to claim
1, wherein at least one of said opposite end pieces includes a
plurality of apertures defined therethrough in communication with
said pocket.
14. The implant for promoting fusion bone growth according to claim
1, wherein said central element is attached to said opposite end
pieces substantially equidistant from each of said two opposite
surfaces of each end piece.
15. The implant for promoting fusion bone growth according to claim
1, wherein said load bearing member includes a center piece having
two opposite surfaces configured to contact the adjacent vertebrae,
said center piece being connected to said central element between
said opposite end pieces and bisecting said pocket.
16. The implant for promoting fusion bone growth according to claim
1, wherein said central element is attached to said opposite end
pieces substantially equidistant from each of said two opposite
surfaces of each end piece.
17. The implant for promoting fusion bone growth according to claim
1, further comprising a side piece spanning between said opposite
end pieces and offset from said central element.
18. The implant for promoting fusion bone growth according to claim
1, wherein said central element includes at least two rods
connected to said opposite end pieces.
19. The implant for promoting fusion bone growth according to claim
1, wherein said central element includes a wall connected to said
opposite end pieces, said wall bifurcating said pocket.
20. The implant for promoting fusion bone growth according to claim
19, wherein said wall includes at least one opening defined
therethrough and communicating with said pocket.
21. The implant for promoting fusion bone growth according to claim
20, wherein said at least one opening is an elongated slot.
22. The implant for promoting fusion bone growth according to claim
1, wherein at least one of said opposite end pieces includes
opposite arc segments, said arc segments defining said two opposite
surfaces contacting the adjacent vertebrae.
23. The implant for promoting fusion bone growth according to claim
22, wherein said opposite arc segments include a plurality of
apertures defined therethrough communicating with the adjacent
vertebrae when said two opposite surfaces are in contact with the
adjacent vertebrae.
24. The implant for promoting fusion bone growth according to claim
1, wherein said elongated central element has a length between said
opposite end pieces, said length being sized to maintain contact
between said opposite end pieces and cortical bone of the adjacent
vertebrae when said load bearing member is in contact with the
adjacent vertebrae.
25. An implant for promoting fusion bone growth in the space
between upper and lower adjacent vertebrae, comprising:
a load bearing member including opposite end pieces and a rigid
elongated central element extending between said end pieces,
said opposite end pieces each having two opposite surfaces
configured to contact and support the upper and lower adjacent
vertebrae, said opposite end pieces sized to maintain the space
between the adjacent vertebrae,
said opposite end pieces each defining a retaining surface
connected to said central element, at least one retaining surface
being concave,
said central element being sized relative to said opposite end
pieces to define a pocket between said central element and the
adjacent vertebrae when the adjacent vertebrae are supported by
said opposite end pieces, said pocket configured to contain an
osteogenic material disposed about said central element and in
intimate contact with the adjacent vertebrae when the vertebrae are
supported by said opposite end pieces.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an implant to be placed into the
intervertebral space left after the removal of a damaged spinal
disc. Specifically, the invention concerns an osteogenic fusion
device that enhances arthrodesis or fusion between adjacent
vertebrae while also maintaining the normal spinal anatomy at the
instrumented vertebral level.
In many cases, low back pain originates from damages or defects in
the spinal disc between adjacent vertebrae. The disc can be
herniated or can be affected by a variety of degenerative
conditions. In many cases, these pathologies affecting the spinal
disc can disrupt its normal anatomical function of the disc. In
some cases, this disruption is significant enough that surgical
intervention is indicated.
In one such surgical treatment, the affected disc is essentially
removed and the adjacent vertebrae are fused together. In this
treatment, a discectomy procedure is conducted to remove the disc
nucleus while retaining the annulus. Since the disc material has
been removed, a body must be placed within the intervertebral space
to prevent the space from collapsing.
In early spinal fusion techniques, bone material, or bone
osteogenic fusion devices, were simply disposed between adjacent
vertebrae, typically at the posterior aspect of the vertebrae. In
the early history of these osteogenic fusion devices, the
osteogenic fusion devices were formed of cortical-cancellous bone
which was not strong enough to support the weight of the spinal
column at the instrumented level. Consequently, the spine was
stabilized by way of a plate or a rod spanning the affected
vertebrae. With this technique, once fusion occurred across and
incorporating the bone osteogenic fusion device, the hardware used
to maintain the stability of the spine became superfluous.
Following the successes of the early fusion techniques, focus was
directed to modifying the device placed within the intervertebral
space. Attention was then turned to implants, or interbody fusion
devices, that could be interposed between the adjacent vertebrae,
maintain the stability of the disc interspace, and still permit
fusion or arthrodesis. These interbody fusion devices have taken
many forms. For example, one prevalent form is a cylindrical hollow
implant or "cage". The outer wall of the cage creates an interior
space within the cylindrical implant that is filled with bone
chips, for example, or other bone growth-inducing material.
Implants of this type are represented by the patents to Bagby, U.S.
Pat. No. 4,501,269; Brantigan, U.S. Pat. No. 4,878,915; Ray, U.S.
Pat. No. 4,961,740; and Michelson, U.S. Pat. No. 5,015,247. In some
cases, the cylindrical implants included a threaded exterior to
permit threaded insertion into a tapped bore formed in the adjacent
vertebrae. Alternatively, some fusion implants have been designed
to be impacted into the intradiscal space.
Experience over the last several years with these interbody fusion
devices has demonstrated the efficacy of these implants in yielding
a solid fusion. Variations in the design of the implants have
accounted for improvements in stabilizing the motion segment while
fusion occurs. Nevertheless, some of the interbody fusion devices
still have difficulty in achieving a complete fusion, at least
without the aid of some additional stabilizing device, such as a
rod or plate. Moreover, some of the devices are not structurally
strong enough to support the heavy loads and bending moments
applied at certain levels of the spine, namely those in the lumbar
spine.
Even with devices that do not have these difficulties, other less
desirable characteristics exist. Recent studies have suggested that
the interbody fusion implant devices, or cages as they are
frequently called, lead to stress-shielding of the bone within the
cage. It is well known that bone growth is enhanced by stressing or
loading the bone material. The stress-shielding phenomenon relieves
some or all of the load applied to the material to be fused, which
can greatly increase the time for complete bone growth, or disturb
the quality and density of the ultimately formed fusion mass. In
some instances, stress-shielding can cause the bone chips or fusion
mass contained within the fusion cage to resorb or evolve into
fibrous tissue rather than into a bony fusion mass.
A further difficulty encountered with many fusion implants is that
the material of the implant is not radiolucent. Most fusion cages
are formed of metal, such as stainless steel, titanium or porous
tantalum. The metal of the cage shows up prominently in any
radiograph (x-ray) or CT scan. Since most fusion devices completely
surround and contain the bone graft material housed within the
cage, the developing fusion mass within the metal cage between the
adjacent vertebrae cannot be seen under traditional radiographic
visualizing techniques and only with the presence of image scatter
with CT scans. Thus, the spinal surgeon does not have a means to
determine the progress of the fusion, and in some cases cannot
ascertain whether the fusion was complete and successful.
The field of spinal fusion can be benefited by an intervertebral
fusion device that can support bone growth material within the
intervertebral space, while still maintaining the normal height of
the disc space. The device would beneficially eliminate the risk of
stress-shielding the fusion mass, and would also provide for
visualization of the fusion mass as the arthrodesis progresses.
SUMMARY OF INVENTION
To address the current needs with respect to interbody fusion
devices, the present invention contemplates a osteogenic fusion
device that is configured to place as much of the bone growth
inducing material as possible into direct contact with the adjacent
bone. In one embodiment, the osteogenic fusion device includes an
elongated body having opposite first and second end pieces
separated by an integral central element. The central element has a
significantly smaller diameter than the two end pieces. The
osteogenic fusion device thus forms an annular pocket between the
end pieces and around the central element.
In accordance with one aspect of the present invention, a bone
growth inducing material is disposed within the annular pocket
around the central element of the osteogenic fusion device. In one
specific embodiment, the bone growth inducing material can
constitute a sheet of a pharmaceutically suitable carrier for a
bone growth factor, such as a bone morphogenetic protein. In this
embodiment, the sheet can be a collagen sheet that is soaked with
the BMP and then subsequently wrapped in spiral fashion around the
central element of the osteogenic fusion device.
In one feature of the present invention, the osteogenic fusion
device can be implanted in a bi-lateral approach. Specifically, two
such osteogenic fusion devices can be inserted into prepared bores
formed in the endplates of adjacent vertebrae after completion of a
discectomy. The spinal loads are borne by the two end pieces that
are in direct contact with the adjacent vertebral bodies.
Preferably, the osteogenic fusion device has a length sufficient to
allow the end pieces to at least partially contact the harder bone
at the apophysis of the adjacent vertebrae. With the osteogenic
fusion device thus inserted, the bone growth inducing material is
in direct contact with the adjacent vertebral bodies. In addition,
bone growth inducing material can be placed within the bi-lateral
space separating the two osteogenic fusion devices. When fusion
occurs, a substantial fusion mass is produced that is virtually
uninterrupted by the material of the osteogenic fusion device
itself.
Several alternative embodiments of the osteogenic fusion device are
presented, all retaining the capability of supporting bone growth
inducing material so that it is in direct contact with the adjacent
vertebrae. In some embodiments, additional elements of the central
element are provided, while in another embodiment, an intermediate
piece is provided for further support across the disc space.
The present invention also contemplates an insertion tool and
certain modifications to the osteogenic fusion device to
accommodate the tool. In one preferred embodiment, the tool is
essentially an elongated shank having opposite prongs extending
therefrom. The prongs can engage truncated side walls of one of the
end pieces. In addition, the opposite end piece can be formed with
notches to receive the tips of the two prongs. With this design,
the osteogenic fusion device can be a push-in or a threaded type
osteogenic fusion device.
It is one object of the present invention to provide an interbody
fusion device that allows the greatest possible contact between the
adjacent vertebrae and the bone growth inducing material supported
by the osteogenic fusion device. It is a further object to provide
such a osteogenic fusion device that is capable of supporting the
loads generated throughout the spine without stress-shielding
developing bone within the osteogenic fusion device.
Another object of the invention is achieved by features that
minimize the radio-opacity of the device. This results in a benefit
to the surgeon of being able to more readily assess the progress of
a spinal fusion.
Other objects and benefits of the present invention can be
discerned from the following written description and accompanying
figures.
DESCRIPTION OF THE FIGURES
FIG. 1 is a top elevational view of a osteogenic fusion device in
accordance with one embodiment of the present invention.
FIG. 2 is an end elevational view of one end of the osteogenic
fusion device shown in FIG. 1.
FIG. 3 is a top elevational view of an alternative embodiment of
the osteogenic fusion device utilizing exterior threads.
FIG. 4 is a top cross-sectional view of a osteogenic fusion device
as shown in FIG. 1 with a bone growth inducing material supported
by the osteogenic fusion device.
FIG. 5 is an cross-sectional view of the osteogenic fusion device
and bone growth material shown in FIG. 4 taken along line 5--5 as
viewed in the direction of the arrows.
FIG. 6 is a plan view of a sheet for a bone growth inducing
material used with the osteogenic fusion device shown in FIG.
4.
FIG. 7 is an end elevational view of one end of a osteogenic fusion
device, such as the osteogenic fusion device of FIG. 1, modified to
include apertures.
FIG. 8 is an end elevational view of one end of a osteogenic fusion
device, such as the osteogenic fusion device of FIG. 1, modified to
include apertures.
FIG. 9 is a side, partially cross-sectional view of an
intervertebral disc space with a osteogenic fusion device according
to FIG. 1 implanted between adjacent vertebrae.
FIG. 10 is a top elevational view of the superior aspect of the
instrumented vertebral level shown in FIG. 9, depicting bilateral
placement of osteogenic fusion devices according to the present
invention.
FIG. 11 is a cross-sectional view of the instrumented vertebral
segment shown in FIG. 10, taken along line 10--10 as viewed in the
direction of the arrows.
FIG. 12 is a top elevational view of a osteogenic fusion device,
such as shown in FIG. 1, with features to permit insertion of the
osteogenic fusion device.
FIG. 13 is an end elevational view of the osteogenic fusion device
shown in FIG. 12.
FIG. 14 is a side elevational view of an insertion tool according
to one embodiment of the present invention.
FIG. 15 is a top elevational view of the insertion tool shown in
FIG. 14.
FIG. 16 is a top elevational view of a osteogenic fusion device for
restoring the lordotic angle between adjacent vertebrae according
to a further embodiment of the present invention.
FIG. 17 is a top elevational view of a osteogenic fusion device
according to a further embodiment of the present invention.
FIG. 18 is a top elevational view of a osteogenic fusion device
according a still further embodiment of the present invention.
FIG. 19 is an end elevational view of the osteogenic fusion device
shown in FIG. 18.
FIG. 20 is a top elevational view of a osteogenic fusion device
according to another embodiment of the present invention.
FIG. 21 is an end elevational view of the osteogenic fusion device
shown in FIG. 20
FIG. 22 is a top elevational view of a osteogenic fusion device
according to yet another embodiment of the present invention.
FIG. 23 is an end elevational view of the osteogenic fusion device
shown in FIG. 22.
FIG. 24 is a top elevational view of a osteogenic fusion device
according to a further embodiment of the present invention.
FIG. 25 is an end elevational view of the osteogenic fusion device
shown in FIG. 24.
FIG. 26 is a top elevational view of a pair of fusion devices
according to FIGS. 24-25 disposed in a bilateral configuration in
the lumbar spine.
FIG. 27 is a top elevational view of a fusion device according to
FIGS. 24-25 disposed in the cervical spine.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
For the purposes of promoting an understanding of the principles of
the invention, reference will now be made to the embodiments
illustrated in the drawings and specific language will be used to
describe the same. It will nevertheless be understood that no
limitation of the scope of the invention is thereby intended, such
alterations and further modifications in the illustrated device,
and such further applications of the principles of the invention as
illustrated therein being contemplated as would normally occur to
one skilled in the art to which the invention relates.
The present invention contemplates osteogenic fusion devices for
use as interbody fusion devices. The osteogenic fusion devices
include opposite end pieces that are configured to span the
intervertebral disc space and engage the adjacent vertebral bodies.
The inventive osteogenic fusion devices include a central element
separating the two end pieces and substantially spanning the
anterior-posterior length of the disc space. The invention further
contemplates that a bone growth-inducing material be disposed about
the central element and between the opposite end pieces. When the
inventive osteogenic fusion device is implanted within a patient,
the bone growth-inducing material is in direct contact with the
adjacent vertebral bodies. The end pieces are formed of a material
sufficient to withstand the spinal loads generated at the
instrumented vertebral level.
In accordance with one embodiment of the invention, an osteogenic
fusion device 10, depicted in FIGS. 1-2, includes a first end piece
11 and a second end piece 12. The end pieces are separated by a
central element 13. The first end piece 11 could be substantially
cylindrical or any geometrical shape and includes an outer bone
contacting surface 15. The end piece 11 also defines an inwardly
facing retaining surface 17. The central element 13 integrally
extends from the retaining surface 17 of the first end piece
11.
The second end piece 12 also defines a bone contacting surface 20
that, in this embodiment, does not extend entirely around the end
piece. The bone contacting surface 20 could be any geometrical
shape, preferably circular and is defined at a radius equal to the
radius of the outer surface 15 of the first end piece. Thus, as
depicted in FIG. 2, the bone contacting surface 20 of the second
end piece 12 is generally coincident with portions of the outer
surface 15 of the first end piece 11 when the osteogenic fusion
device is viewed along the longitudinal axis of its central element
13. The second end piece 12 also includes opposite truncated
surfaces 21 that are disposed between the circular bone contacting
surfaces 20. Preferably, the truncated surfaces 21 are generally
flat and can be configured to be engaged by an insertion tool. The
insertion tool preferably has arms that contact the flat truncated
surfaces 21, yet still fall within the envelope defined by the
outer surface 15 of the first end piece 11.
The second end piece 12 also defines a second retaining surface 22
that faces the first retaining surface 17 of the first end piece
11. Again, the central element 13 is preferably integral with and
projects outwardly from the second retaining surface 22.
Alternatively, the central element can be in the form of a central
rod that is engaged within colinear bores formed in the two end
pieces. In this variation, the engagement between the central rod
and the end pieces can be threaded.
The central element 13 includes an outer central surface 23.
Preferably, the central element 13 is substantially cylindrical
along its length. In one aspect of the invention, the first end
piece 11 defines a diameter D.sub.1, while the central element 13
defines a diameter D.sub.2. The diameter D.sub.1 is at least equal
to the height of the intervertebral space within which the
osteogenic fusion device 10 is to be interposed. Most preferably,
the diameter D.sub.1 corresponds to the diameter of a cylindrical
channel cut into the endplates of the adjacent vertebrae. In this
instance, the diameter D.sub.1 will be somewhat larger than the
intervertebral disc space height. Moreover, the diameter D.sub.1 is
significantly larger than the diameter D.sub.2 of the central
element 13. This diameter differential creates an annular pocket 24
surrounding the central element 13.
The osteogenic fusion device 10 has a length L.sub.1 between the
opposite ends of the osteogenic fusion device. This length L.sub.1
is preferably selected to be slightly less than the
anterior-posterior length of the intervertebral disc space,
although the length can be calibrated to the lateral dimension of
the space. Most preferably, the length L.sub.1 is sized so that the
first and second end pieces 11, 12 can contact at least a portion
of the apophysis or harder cortical bone at the perimeter of the
vertebral endplates. The osteogenic fusion device 10 further
defines a length L.sub.2 which is essentially the length of the
central element 13. The length L.sub.2 is calibrated so that the
end pieces 11 and 12 are sufficiently wide to provide adequate
support between the adjacent vertebrae. Conversely, the length
L.sub.2 is sufficiently long so that the annular pocket 24 has the
capacity for retaining a substantial quantity of bone
growth-inducing material.
In a modification of the osteogenic fusion device 10, the second
end piece can be configured with threads. For example, as depicted
in FIG. 3 an end piece 25 includes external bone engaging threads
26 extending from the outer surface 27. In accordance with this
embodiment, the second end piece 25 can be cylindrical, like the
first end piece 11, or the threads can be formed between truncated
surfaces, such as truncated surfaces 21 in the prior embodiment. At
any rate, the threaded end piece 25 is configured to be threadedly
advanced into a drilled and tapped channel within the adjacent
vertebral bodies. The first end piece 11 can also be threaded to
facilitate insertion and to reduce the chance of expulsion.
In a further aspect of the invention, a bone growth inducing
material 30 is provided for support by the osteogenic fusion device
10. Preferably the material 30 is in the form of a sheet. In a
specific example, the carrier sheet 30 can be a collagen sheet that
is soaked with a solution containing a bone growth inducing
substance, or a bone morphogenetic protein (BMP). In accordance
with the invention, the carrier sheet 30 can be formed of a variety
of materials other than collagen, provided the materials are
capable of containing a therapeutically effective quantity of a
bone growth inducing substance or BMP. Moreover, the material 30,
whether in sheet form or not, is most preferably susceptible to
manipulation to be disposed within the annular pocket 24 of the
fusion device 10.
In accordance with the specific embodiment, the carrier sheet 30 is
wound around the outer surface 23 of the central element 13 (see
FIG. 5). The carrier sheet 30 is held between the retaining surface
17 of the first end piece 11 and the retaining surface 22 of the
second end piece 12. In accordance with one specific embodiment,
the retaining surface 22 is curved or convex. In this way, the
carrier sheet 30 can project into the convexity to serve as a sort
of anchor to hold the carrier sheet 30 within the annular pocket 24
of the osteogenic fusion device 10. In addition, the convex surface
22 conforms better with the anterior portion of the vertebral body
profile when the fusion device is implanted.
In the illustrated embodiment, the carrier sheet 30 can be provided
as a single sheet, as shown in FIG. 6. The inner end 31 of the
sheet is disposed against the central outer surface 23 of the
central element 13. The sheet can be wound in a spiral fashion
about the central element 13 until its outer end 32 is disposed
adjacent the outer surface 15 of the first end piece 11. The
carrier sheet 30 has width W that is preferably slightly larger
than the length L.sub.2 between the first and second end pieces to
allow a portion of the carrier sheet 30 to project into the concave
retaining surface 22 of the second end piece 12. The overall length
of the sheet 30 between ends 31 and 32 depends upon its thickness
and the difference in diameters D.sub.1 and D.sub.2. For example,
in one embodiment the diameter D.sub.2 is about one-fourth (1/4)
the diameter D.sub.1. Preferably, the length is sufficient so that
the carrier sheet 30 can be tightly wound about the central element
13 and fill the annular pocket 24. One important object of the
present invention is that the carrier sheet 30 or bone growth
inducing material reside in direct contact with the adjacent
vertebral bone. Consequently, the sheet 30 is preferably wound so
that its outer end 32 is at least slightly outside the envelope of
the outer surface 15 of the first end piece 11.
The carrier sheet 30 of FIGS. 4-6 illustrates one specific
embodiment of bone growth-inducing material usable with the
osteogenic fusion device of the present invention. It is also
contemplated that the carrier can be in the form of a sponge,
paste, gel or a settable osteogenic material. The osteogenic
material must be provided in some form that can be generally
retained about the central element 13 and within the annular pocket
24 of the osteogenic fusion device 10. Put differently, the present
invention contemplates an osteogenic material that does not need to
be contained in the traditional manner of the hollow cylindrical
cages of the prior art. In these prior art devices, cancellous bone
chips have been contained within a hollow cage. The present
invention does not contemplate the use of bone chips alone.
However, bone chips contained within a bone paste or a gel may be
utilized with the osteogenic fusion device 10, provided that the
paste or gel have a consistency sufficient to hold the bone growth
inducing material on and within the osteogenic fusion device
10.
In accordance with one specific embodiment, the end pieces 11 and
12 are solid and circular in configuration. Alternative end piece
configurations are shown in FIGS. 7 and 8. For example, end piece
11' can have a plurality of generally circular apertures 34
disposed circumferentially about the end piece, as shown in FIG. 7.
The end piece 11" shown in FIG. 8 includes a plurality of
pie-shaped apertures 35 so that the end piece gives the appearance
of a spoked wheel. The second end piece 12 of the osteogenic fusion
device 10 can have similar apertures defined therethrough. The
apertures 34 and 35 in the end pieces 11', 11" provide a further
avenue for facilitating fusion bone growth. The apertures
themselves can be filled with a osteogenic material, such as a gel
or a paste. Moreover, once the osteogenic fusion device 10 is
implanted within an intervertebral disc space, osteogenic material
can be packed around the osteogenic fusion device within the disc
space. These additional apertures in the end pieces 11, 12 provide
further avenues for the formation of a bony bridge between adjacent
vertebrae.
The end pieces 11,12, etc. can also have non-circular shapes. For
instance, the end pieces can be rectangular or other multi-sided
shapes. If the osteogenic fusion device resides within a channel
prepared in the endplates, the channel shape can be modified to
conform to the bone engaging surfaces 15, 20 of the end pieces.
FIGS. 9-11 depict a pair of osteogenic fusion devices 10 implanted
in a bi-lateral configuration between adjacent vertebral bodies
V.sub.1 and V.sub.2. As depicted, the disc annulus A is retained
but at least one portal must be defined in the annulus A to permit
insertion of the osteogenic fusion devices 10. The present
invention also contemplates insertion of each osteogenic fusion
device 10 through its own portal formed in the disc annulus A.
Alternatively, in conformance with other known procedures, a single
portal can be provided through which each osteogenic fusion device
10 is successively inserted. Further in accordance with the present
invention, the osteogenic fusion devices 10 can be positioned
within the intervertebral disc space according to known posterior
or postero-lateral techniques.
According to the present invention, the osteogenic fusion device 10
is inserted into the disc space S with the first end piece 11
proceeding first into the space. Preferably, a channel C is bored
into the vertebral endplates E to a preferred depth of insertion of
the osteogenic fusion device 10, in accordance with known
techniques. If the osteogenic fusion device to be implanted is of
the type shown in FIG. 3 with the threaded second end piece 25, the
channels C can be appropriately drilled and tapped to accommodate
the bone engaging threads 26. In a modification of this embodiment,
the first end piece 11 can also carry external threads.
The preferred embodiment contemplates a generally cylindrical
osteogenic fusion device placed within circular channels.
Alternatively, the osteogenic fusion devices can operate as spacers
that directly contact the endplates, without a prepared channel. In
this instance, the bone engaging surfaces of the end pieces can be
modified to conform to the vertebral endplate geometry.
As depicted in FIGS. 9-11, the osteogenic material 30 is disposed
in direct contact with the adjacent vertebral endplates E.
Moreover, the placement of osteogenic fusion devices 10 can present
a medial space 37 between the two osteogenic fusion devices.
Osteogenic material can then be placed within the medial space 37,
again in direct contact with the osteogenic material 30 situated
around the central elements 13 of each of the osteogenic fusion
devices 10. Once complete fusion occurs, new bone growth will
substitute the carrier material 30 to form a solid bony bridge
spanning the adjacent vertebrae V.sub.1, V.sub.2. As can be seen
from FIGS. 9-11, the region of continuous bone growth is very
substantial and is not interrupted by the structure of the fusion
device itself.
It is understood, of course, that the end pieces 11 and 12 provide
sufficient support for the vertebral loads passing between the
adjacent vertebrae. At the same time, this load bearing capacity is
concentrated outside the middle regions of the vertebral endplates
E. It is known that the central region of the endplates is very
rich in blood flow and has a high capacity for new bone growth.
Thus, the elimination of structural material of the osteogenic
fusion device 10 from that region provides for a faster and more
complete arthrodesis than may have been possible with prior fusion
cages.
Referring next to FIGS. 14, 15, an insertion tool 50 is depicted
for inserting a osteogenic fusion device 10 according to the
present invention. The insertion tool 50 includes a solid shank 51
to which a knob or handle 52 is affixed. The knob 52 is configured
for manual grasping and manipulation during insertion of the
osteogenic fusion device. In the case where the osteogenic fusion
device is not threaded, the insertion tool 50 simply acts as a
pushing device. On the other hand, in the instance where the
osteogenic fusion device includes threaded end pieces such as shown
in FIG. 3, the insertion tool 50 must be rotated as the end piece
is threaded into the prepared channel between the adjacent
endplates.
The insertion tool 50 includes a pair of prongs 53 that are
disposed apart to define an end piece recess 54. For insertion of
the osteogenic fusion device 10 shown in FIG. 1, the end piece
recess 54 is configured so that the prongs 53 are in tight contact
with the truncated surfaces 21 of the second end piece 12. The
outer surface of the prongs 53 can conform to a portion of the
outer surface 15 of the first end piece 11.
The insertion tool 50 depicted in FIGS. 14-15 also includes tapered
tips 55 at the ends of each of the prongs 53. These tapered tips
are configured to be received within driving notches 41 in a
modified first end piece 40, as depicted in FIGS. 12-13. The
osteogenic fusion device depicted in FIGS. 12-13 is substantially
similar to the osteogenic fusion device 10 shown in FIG. 1, with
the exception of the added driving notches. The insertion tool 50
is configured so that the tips 55 project into the notches 41 while
the prongs 53 directly contact the truncated surfaces 21 of the
second end piece 12. This particular configuration of the insertion
tool is particularly useful for threaded insertion of the
osteogenic fusion device. Preferably, the prongs 53 have an
effective outer diameter that is approximately equal to the
diameter D.sub.1. Moreover, the prongs 53 can have an arc segment
configuration to complement the truncated surfaces 21. If the end
piece 12 is threaded (see FIG. 3), the prongs 53 can include
complementary threads along their length.
The present invention also contemplates a osteogenic fusion device
for restoring the normal lordotic angle of an intervertebral
segment. Specifically, a lordotic osteogenic fusion device 60
includes a first end piece 61 and a second end piece 62 as shown in
FIG. 16. As with the prior embodiments, a central element 63 is
provided to connect the two end pieces. The outer surface 65 of the
first end piece 61 is in the form of a frusto-conical surface. The
outer surface 65 tapers toward the second end piece 62 at a
preferred lordotic angle. Similarly, the outer surface 66 of the
second end piece 62 is also tapered at a similar lordotic angle.
Alternatively, the second end piece 62 can include threads formed
on the outer surface 66. While the threads 66 at the smaller second
end piece 62 may not contact the vertebral endplates at the larger
insertion end, the threads will contact the endplates at the
anterior end of the intradiscal space and will act as an anchor to
resist expulsion of the lordotic osteogenic fusion device 60.
The present invention contemplates several modifications to the
basic osteogenic fusion device 10. For example, the osteogenic
fusion device 70 shown in FIG. 17 includes first and second end
pieces 71, 72 and a center piece 73 disposed between the two end
pieces. First and second central elements 74 and 75 connect each of
the end pieces 71, 72 to the center piece 73. In this instance, the
center piece 73 will contact the interior of the disc endplates E.
Osteogenic material, such as carrier sheets 30, can be disposed or
wound around each of the central elements 74, 75 until the end of
the bone growth inducing material is exposed at the outer surface
of the osteogenic fusion device 70.
In a further modification, a osteogenic fusion device 80 depicted
in FIG. 18 includes first and second end pieces 81 and 82 that are
connected by a plurality of central beams 83. In the illustrated
embodiment as shown in FIG. 19, four such beams 83 are provided;
however, other arrangements and numbers of beams are contemplated.
Important aspects of the present invention are retained by the
osteogenic fusion device 80 because osteogenic material can be
supported by the several beams 83 between the first and second end
pieces 81, 82, with the bone growth inducing material in direct
contact with the adjacent vertebral bodies.
The two embodiments of FIGS. 20-21 and FIGS. 22-23 pose a slight
deviation from the general concept of the osteogenic fusion device
10. In these two embodiments, the smaller diameter central element
13 is replaced by a wall. In the embodiment of FIGS. 20-21, a
osteogenic fusion device 85 includes first and second ends 86, 87
separated by a central element 88. The first and second ends 86 and
87 can be in the form of short cylindrical sections, such as the
first end piece 11 of the osteogenic fusion device 10 in FIG. 1.
While the central element 88 can be in the form of a solid wall,
the osteogenic fusion device 85 preferably includes a number of
slots 89 defined through the central element 88. In accordance with
the specific embodiment, the slots extend along substantially the
entire length of the central element 88. While the osteogenic
fusion device 85 deviates somewhat from the concept of the
osteogenic fusion device 10, this latter osteogenic fusion device
85 retains the broad beneficial feature of the present invention,
namely provision for direct contact between osteogenic material
supported by the osteogenic fusion device 85 and the vertebral
endplates. In the present instance, the osteogenic material can be
situated on opposite sides of the central element 88. In addition,
the material can be passed through the slots 89.
Preferably, the osteogenic fusion device 85 will be oriented within
the intervertebral disc space with the central element 88, or wall,
spanning between the adjacent vertebrae. This central element 88,
then, will provide additional structure and load bearing capability
for sustaining the spinal loads at the instrumented level.
The osteogenic fusion device 90 of FIGS. 22-23 operates on a
similar concept to the osteogenic fusion device 85. However, in
this instance, the first and second end pieces are in the form of
arc segments, rather than shortened cylinders. Specifically, the
osteogenic fusion device 90 includes upper and lower first arc
segments 91.sub.U and 91.sub.L, and upper and lower second arc
segments 92.sub.U and 92.sub.L. The osteogenic fusion device 90
also includes a central element 93 that is again in the form of a
wall connecting the first and second end pieces. As can be seen
most clearly in FIG. 23, the arc segments 91, 92 and central
element 93 define a pair of cavities 96 for containing osteogenic
material. In this embodiment, the osteogenic material can be
contained completely from end to end of the osteogenic fusion
device 90. In the prior embodiments, the osteogenic material is
contained within retaining surfaces of the opposite end pieces. In
accordance with a specific embodiment, the osteogenic fusion device
90 includes a plurality of apertures 94 defined in each of the
upper and lower first and second arc segments 91.sub.U, 91.sub.L,
92.sub.U and 92.sub.L. Similarly, a plurality of apertures 95 can
be defined through the central element 93. In this manner, the
apertures provide the maximum capacity for bone ingrowth not only
around, but also through the osteogenic fusion device 90.
A osteogenic fusion device 100 shown in FIGS. 24-25 again presents
a slightly different concept. This osteogenic fusion device 100
includes a first end plate 101, a second end plate 102 and a
central element 103 that are similar to the like-named components
of the osteogenic fusion device 10. However, the osteogenic fusion
device 100 also includes a side piece 104 spanning between the
first and second end pieces 101, 102. Moreover, unlike the
osteogenic fusion device 10, the first and second end pieces 101,
102 are not generally circular in configuration, but are generally
rectangular in configuration. In one specific embodiment, the end
pieces 101, 102 can include cut outs 105 at opposite sides of the
end pieces to provide further avenues for the formation of a bony
bridge between adjacent vertebrae. As with the prior embodiments,
the osteogenic fusion device 100 provides means for adequately
containing osteogenic material, such as in the form of the carrier
sheet 30. In this embodiment, the carrier sheet 30 can be wound
around the central element 103, in the manner described above. This
particular embodiment of the invention, namely osteogenic fusion
device 100, is preferably adapted for use in the lumbar spine as
illustrated in FIG. 26 and in the cervical spine illustrated in
FIG. 27, and is consequently sized accordingly.
The present invention contemplates osteogenic fusion devices that
are formed of a material that is sufficiently strong to support the
adjacent vertebrae and to maintain the disc height of the
instrumented intervertebral space. For example, the osteogenic
fusion devices, such as osteogenic fusion device 10, can be formed
of a biocompatible sterilizable metal, such as stainless steel or
titanium. Of course, other medical grade materials are
contemplated, such as certain ceramics, polymers, etc., as well as
allograft and xenograft bone, provided the materials are
sufficiently strong. The overall dimensions of each of the
osteogenic fusion devices described above depends upon the
instrumented level. For example, a osteogenic fusion device for use
in the cervical spine must necessarily be smaller than a osteogenic
fusion device used in the lumbar spine. Moreover, the relative
dimensions of the components of the osteogenic fusion devices may
be altered depending upon the vertebral level to be instrumented.
For example, a osteogenic fusion device, such as osteogenic fusion
device 10, for use in the lumbar spine, may require a central
element 13 having a diameter D.sub.2 that is more than one fourth
of the outer diameter D.sub.1 of the outer surface 15 of the first
end piece 11. In some instances, the lumbar spine may generate
bending moments across a osteogenic fusion device, such as
osteogenic fusion device 10, that would require a stronger central
element 13.
In accordance with the present invention, the illustrated
osteogenic fusion devices can be of the push-in or threaded-in
type. Of course, the end pieces, such as end pieces 11, 12 of
osteogenic fusion device 10, can include various surface
characteristics known in the art for enhancing the degree of
fixation of the osteogenic fusion device between the adjacent
vertebrae. For example, the end pieces can include certain macro
surface features for penetrating the vertebral endplates to resist
expulsion of the osteogenic fusion devices. Likewise, the surfaces,
such as outer surface 15 and bone contacting surface 20 can be
provided with bone ingrowth coatings so that a certain amount of
bone ingrowth occurs even between the end pieces and the adjacent
vertebral bodies.
While the invention has been illustrated and described in detail in
the drawings and foregoing description, the same is to be
considered as illustrative and not restrictive in character, it
being understood that only the preferred embodiments have been
shown and described and that all changes and modifications that
come within the spirit of the invention are desired to be
protected.
* * * * *